The present invention relates to the field of telephony, and more particularly ring generator circuitry for telephone systems.
All telephones require an alerting signal or ring signal for notifying a subscriber of an incoming phone call. Early telephones employed mechanical bells that rang in response to an electrical ringing signal appearing on the telephone line. The mechanical bells required a low frequency, high voltage AC signal for ringing. The amplitude of the ring signal had to be relatively high, for example at least 45 volts AC at the phone, and the frequency needed to be quite accurate to ring the phone without fail. Typically, the ring signal is superimposed on a DC voltage that enables the circuitry to detect when the customer goes “off hook” or answers the telephone. Backward compatibility requirements have kept the characteristics of the ringing signal essentially the same over the last century.
The line carrying the ring signal to a telephone instrument is connected to a ring generator when a ring signal is needed and to a talk battery voltage source otherwise. Thus telephone ring signal lines are connected to relays with additional logic to switch the ring generator and the talk battery voltage source onto them for each telephone instrument.
The relays used in telephone circuits are degraded by the action of switching while the high voltages used in the ring generator are across the contacts. The contacts suffer from arcing, pitting and other effects that reduce their useful life. Therefore, with today's systems it is sometimes useful to have the relays switch when the voltage is at or near zero volts as opposed to the −48 volts of the talk battery. This however requires that the switching action take place at unsymmetrical points on the voltage waveforms.